Electrolytic process for obtaining chlorine in a pure state and alkali metal phosphates in concentrated solution and a cell for accomplishing this process

ABSTRACT

A process and apparatus for the manufacture of pure chlorine, hydrogen and alkali metal phosphates in concentrated solution by introducing an aqueous alkali metal chloride solution into the anodic compartment of a cell and concentrated phosphoric acid or alkali metal phosphate aqueous solution into the cathodic compartment. The cell has a cationic permselective membrane placed between a porous anode of the titanium group having a platinum metal group coating on the surface opposite the membrane and the cathode.

UnIted States Patent [191 [11] 3,763,005

Butre et al. 1 Oct. 2, 1973 [54] ELECTROLYTIC PROCESS FOR 3,242,059 3/1966 Cottam et al 204/128 OBTAINNG CHLQRINE IN A PURE STATE 3,179,579 4/1965 Heinemann at al..... 204/128 X AND ALKALI METAL PHOSPIATES IN 3,438,879 4/1969 Kircher et all 204/128 X CONCENTRATED SOLUTION AND A CELL FOREIGN PATENTS OR APPLlCATlONS FOR ACCOMPUSMNG THIS PROCESS 728,627 2/1966 Canada 204/128 [75] Inventors: Jean-Louis Butre; Frangois Pierrot,

both of Lyon France Primary Examiner-F. C. Edmundson [73] Assignee: Progil, Paris, France 4 y- Reed [22] Filed: May 12, 1971 21 Appl. No.: 142,672 [571 {*BSTRACT A process and apparatus for the manufacture of pure {30 Foreign Appncation p o Data chlorine, hydrogen and alkali metal phosphates in con- May 15 I970 France 7017742 centrated solution by introducing an aqueous alkali metal chloride solution into the anodic compartment of I [52] Us. U 204/90 204/128 204/257 a cell and concentrated phosphoric acid or alkali metal [51] Inn CL c'olb 11/26 Bolk 1/00 phosphate aqueous solution into the cathodic compart- [581' Field of Search 204/92) 128 257 ment. The cell has a cationic permselective membrane placed between a porous anode of the titanium group [56] References Cited having a platinum metal group coating on the surface UNITED STATES PATENTS opposite the membrane and the cathode. 3,017,338 1/1962 Butler et a1. 204/128 '7 Claims, 1 Drawing Figure BRINE PURIFICATION 22 0ND 3 FILTEER v Pl Q 1 0,0 a o i I z I 3 42 I, a lo 5" i PUREU :3 5001 M PRECIPITATED PHOSPHATES PHOSPHATES 1 X CRUDE WET PROCESS SATURATOR NuCl PHOSPHORIC ACID PATENTED 2|975 3.763.005

BRINE PURIFICATION 22 PURIFIED 38 FILTER BRINE am v Pt PURE SODIUM PRECIPITATED .0 PHOSPHATES PHOSPHATES l m 26 8 l4 U CRUDE WET PROCESS SATURATOR PHOSPHORIC ACID 9% NuCl INVENTORS JEAN'LOUUS BUTRE FRANCOIS PIERROT ATTORNEYS ELECTROLYTIC PROCESS FOR OBTAINING CHLORINE IN A PURE STATE AND ALKALI METAL PHOSPHATES IN CONCENTRATED SOLUTION AND A CELL FOR ACCOMPLISIIING THIS PROCESS The present invention relates to a process ofmanufacturing electrolytically in a cationic permselective membrane cell, pure chlorine, hydrogen and alkali metal phosphates in concentrated solution, from alkali metal chlorides and phosphoric acid obtained from attack on natural phosphate by the wet process. It relates also to electrolysis equipment for use in this process.

It is known that alkali metal chloride electrolysis in a cell having a permselective membrane of cationexchanger type, gives rise to chlorine, hydrogen and a pure alkali metal hydroxide as compared to cells with diaphragms which act as percolators and which give chlorine, hydrogen and a diluted mixture of alkali metal hydroxides and chlorides.

The use of the large quantities of alkali metal hydroxide produced has always been a troublesome problem. In common practice, thehydroxide has been drawn off, used as it is or neutralized, this neutralization operation being a long and expensive one. A process has been sought for utilizing the hydroxide in the electrolysis cell in order to obtain directly and at a smaller cost, a more valuable product. Thus, a current of carbon dioxide has been introduced into the cathodic compartment of the cell in order to react with the pure sodium hydroxide and give directly sodium carbonate. French Pat. No. 1,094,082 filed on Dec. 16, 1953 describes a process of this type, comprising two permselective membranes delimiting a compartment in which carbon dioxide is introduced in an aqueous medium. This interesting method has the disadvantage however of leading to diluted products which mustthereafter be concentrated which is an expensive operation.

The process according to the present invention, which is much simpler, is based upon the utilization of only one cationic permselective membrane and of an anode having special characteristics, the conjunction of those two elements yielding chlorine and hydrogen,

both in very pure state, and pure alkali metal phosphates in concentrated solution.

The process accordingto the present invention comprises introducing in forced circulation into the anodic compartment an aqueous alkali metal chloride solution, while concentrated phosphoric acid or a concentrated alkali metal phosphate aqueous solution is fed in forced circulation into the cathodic compartment of an electrolysis cell, the cell having a cationic permselective membrane placed between the two electrodes and an anode which is of a porous nature in order to allow electrolyte percolation, and which presents an electrochemical activity only on the face opposite to the membrane.

Because solutions are fed in forced circulation in each compartment, the products resulting from electrolysis and from acid-hydroxide reaction are removed continuously from phosphoric the cell. Thus on the anodic side, a forced flow of gaseous chlorine and of unreacted alkali metal chloride solution goes out, while on the cathodic side a forced flow of gaseous hydrogen and of alkali metal phosphates as concentrated aqueous solution goes out, the gases and liquids being then separated.

pensive operation. This dilution may even be suppressed by a convenient choice of concentration ratio of the two liquid flows.

The permselective membrane is a strong acid cationexchanger-membrane the material of which is chosen from those polymers presenting a good resistance to chlorine and alkaline agents, especially perfluorinated polymers. Substitution resistance of such a membrane :in 0.6 N potassium chloride is between 0.2 and 8 ohm.cm according to the cases, and with a selectivity greater than percent.

Because of the percolation coming from the porous nature of the anode, the anodic compartment is divided into twodistinct portions; the properly anodic portion in which chlorine escapes on the side of the anode active face, and the portion between the anode and the membrane in which the electrolyte continuously sweeps the membrane, which is practically free from reaction products which escape on the active opposite face of the anode. Percolation avoids an undesirable return of chlorine on the membrane, which could be a nuisance during a long operation. The chlorine produced is not. contaminated with any other element; it is practically pure and contains a very small quantity of oxygen. There is no further-formation of oxidized derivatives of chlorine, especially chlora'tes. The anode is a metal anode perforated with fine holes, made of a metal which may be titanium, tantalum, zirconium, niobium or, their alloys and covered only on one face with an active coating which may be platinum, iridium, palladium, ruthenium, osmium, rhodium, their alloys or their oxides. The cathode is a metal cathode which may be perforated.

The principle of a forced circulation of anolyte and catholyte, allows a degassing, outside the cell, of gases formed during electrolysis. Thus this apparatus avoids the various overpressures observed during a natural degassing. Consequently there is no degasing internalcompartment; the mixtures of gas/liquid are separated in external degassers, some liquidl elements being recycled after purification.

The 'problems of oxidizing corrosion of the permselective membrane are quite reduced becausephosphoric acid is present in the cathodic compartment. The membrane is in contact with an acid or weakly basic medium, contrary to the classical case in which concentrated sodium hydroxide solution is present in the cathodic compartment.

The solution of alkali metal chlorides preferably has a concentration near to saturation. For example, there may be used, in the case of sodium chloride solution, an industrial brine containing 300-315 g NaCl/l. The solutionmay also be continuously resaturated; However it is possible to use solutions having a lower concentration for example down to 50 g/l.

The phosphoric acid used is the crude acid coming from treatment of natural phosphate by the wet process, and containing various cationic impurities. Since this acid is continuously introduced into the cathodic compartment, some of the metals present are reduced by the escaping hydrogen and precipitate in the from of phosphates in which the metal displays alow valency, which facilitates continuous filtration and a substantial recovery of P,O,, in the form of alkali metal phosphates. A complete removal of chromium and vanadium results, such products being especially troublesome in the further preparation of white alkali metal tripolyphosphates. There is removed from the cell a continuous flow of gaseous hydrogen, precipitated metal phosphates, which are continuously filtered thereafter, and a concentrated solution of alkali metal phosphates. There is practically no pollution of the phosphates by sodium chloride, nor of sodium chloride by phosphates, which eliminates'the purification and concentration circuits required in the classical processes.

instead of crude wet process phosphoric acid, it is also possible to introduce, as a variant, pure concentrated phosphoric acid or concentrated solutions of alkali metal phosphates having a given ratio Na/P. There is produced in this last case concentrated alkali metal phosphates having a higher ratio Na/P. In a general way, the P content of the solution may vary between 200 and 500 g/l.

The electrical energy used in the process according to the present invention is lower than that necessary for the production of the same quantity of chlorine in diaphragm cells. Electrical yield is greater than 99.5 percent.

Chlorine free from hydrogen (0.02 percent) contains less than 0.5 percent of oxygen.

Hydrogen is pure.

The alkali metal phosphates produced contain less than 0.3 percent of the corresponding chloride.

There is no phosphorus loss by migration into the anolyte (less than parts per million in equilibrium state).

The attached drawing will allow a better understanding of how the invention may be put into practice. In the single FIGURE, brine containing the recycled salt thru line 8 and a fresh quantity of salt thru 9 arrives continuously in cell 10 thru line 11 between the membrane l2 and the anode 14.

Brine sweeps the membrane 12 which lets the sodium ions pass and electrolysis is conducted on the platinum covered active side 16 of the perforated titanium anode 14. Because of the circulation, very pure chlorine is removed at 18 at the same time as the unreacted brine. The mixture passes thru one or several degassers 20 which completely remove chlorine, which is sent to storage and brine is recycled thru 24 and a purifier 25. Phosphoric acid is fed continuously thru 26 into the cathodic compartment 28 consisting of a perforated nickel cathode 30. Because of the circulation, hydrogen, concentrated alkali metal phosphates and the precipitate of other metal phosphates are removed continuously thru 32, then pass into a degasser 34. Hydrogen is sent to storage thru 36, the precipitated phosphates are filtered at 38 and removed at 40 and pure alkali metal phosphates are obtained at 42 as a concentrated aqueous solution.

The cell described in this way is a monoelement cell. Several monoelements may be mounted in series, according to techniques similar to that of filter-presses.

The following examples will give, in a non limitative way, illustrations of the invention.

EXAMPLE 1 sodium chloride 304 gr. per liter sodium carbonate 0.3 gr. per liter sodium sulfate 7 gr. per liter Ca l.5 gr. per liter Mg 0.4 gr. per liter In the cathodic compartment there is introduced by pumping, a crude phosphoric acid coming from natural phosphate attach by the wet process, and containing especially:

P,0, 310 gr. per liter H,SO 10 gr. per liter SiO, 0.5 gr. per liter CaO and MgO 8 gr. per liter AI,O, and Fe,0, 9 gr. per liter Cr 0.15 gr. per liter V 0.15 gr. per liter The balance, after running for 1,000 hours, without any notable variation of the various characteristics was as follows:

60 to 65C.

Cell temperature 0. l3 ampere/cm Current density at membrane Cell voltage 4 volts Anolyte recycling rate Anolyte impurity 10 ppm of phosphorus The products obtained, of very homogeneous quality, had the following composition:

Gaseous mixture from the anodic compartment:

chlorine 99% CO: 0.5% 0.5% v H, 0.02% Gas leaving the cathodic compartment pure H, Liquid mixture leaving the same compartment, after filtration:

P205, 270 gll.

ratio Na/P L66 corresponding to basis product of sodium tripolyphosphate manufacture C30 and MgO 0.12 g. for 100 g. of P,O,

NaCl- 0.5% in P,O,

EXAMPLE 2 in the cell used in example I, there was brought into the anodic compartment, between the anode and the membrane, an industrial brine of the same composition as that in Example 1. In the cathodic compartment pure phosphoric acid with 240 g/l. of P,(), was introduced.

The balance, after running for L000 hours, without any notable variation of the various characteristics, was as follows:

60C. 0.13 ampere/em Cell temperature Current density at membrane Cell voltage Anolyte recycling rate Anolyte impurities 3.8 volts 1 75% 10 ppm of phosphorus There was obtained a monosodium phosphate having 220 g/l. of P a ratio Na/P 1.25, and with less than 0.2 percent by weight of sodium chloride. The hydrogen from the cathodic compartment was pure.

Gases evolved in the anodic compartment had the following composition:

Chlorine 99% CO, 0.5% O, 0.5% H, 0.02%

EXAMPLE 3 Temperature 60C. Voltage 4 volts Anolyte recycling rate 70% EXAMPLE 4 The solution arriving continuously under pressure into the anodic compartment of the cell used in the previous examples was potassium chloride of 340, g/l.

Crude wet process phosphoric acid was introduced into the cathodic compartment having 300 g/i. of P 0 There was obtained a production of potassium phosphate with a ratio K/P==l in an aqueous solution of 280 g/l. The gases formed had the same composition as in example 1.

The characteristics of cell running were:

Temperature Voltage EXAMPLE 5 In the same cell a solution of pure sodium chloride was fed into the anodic compartment, resaturated continuously.

Anolyte concentration was maintained near 310 g/l. of sodium chloride.

In the cathodic compartment there was introduced pure phosphoric acid with 300 g/l. in P 0 The characteristics of cell running were the following:

Temperature Voltage It was possible to obtain sodium phosphate having a ratio Na/P 1.66 for 300 g/l. in P 0 The purity of the various products was identical to that of the previous examples.

It will be obvious to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown in the drawing and described in the specification.

What is claimed is:

1. A process of manufacturing chlorine in a pure state, hydrogen, and alkali metal phosphates in concentrated solution in an electrolysis cell, said cell having a cationic permselective membrane separating an anode from a cathode and defining, respectiveiy, anodic and cathodic compartments, said anode being porous and presenting an electrochemical activity only on its face opposite from that nearest to said cationic permselective membrane, which process comprises introducing in forced circulation into the anodic compartment between the anode and said cationic permselective membrane an aqueous alkali metal chloride solution, at the same time introducing concentrated phosphoric acid or a concentrated alkali metal phosphate aqueous solution in forced circulation into the cathodic compartmerit, forcing anolite electrolyte to percolate through the porous anode thereby preventing chlorine from escaping into the space between said anode and said cationic permselective membrane.

2. A process according to claim 1 in which said phosphoric acid is the crude acid coming from natural phosphate attack by the wet process.

3. A process according to claim 1 in which an alkali metal phosphate in concentrated solution is introduced into the cathodic compartment, whereby as a result of the process the alkali metal to phosphorus ratio is enhanced.

4. A process according to claim 1 in which a mixture of chlorine and a solution of alkali metal chlorides, is continuously removed from said anodic compartment.

5. A process according to claim 1 in which a mixture of hydrogen and of phosphates in concentrated state is continuously removed from said cathodic compartment.

6. A process according to claim lin which said cationic permselective membrane is made of a polymer presenting a good resistance to chlorine and alkaline agents.

'1. A process according to claim 1 in which said anode comprises a metal of the titanium group, coated only on one face with an active coating of a noble metal of the platinum group or of an oxide or alloy thereof.

F4050 UNITED STATES PATENT ()FFHIE CERTIFICATE OF coREcTroN Patent No. 3,7 3, 5 Dated October 2. 1973 -Inventor(s) Jean-Louis Butre and Francois Pierrot It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 61, cancel the word "phosphoric" after the word "from" and insert it in line 60,- after the word "from".

Column 3, line 3, the word "from" should be "form".

Column a, line 22, the ward "attach" should be "attack".

Signed and sealed this 5th day of February 1974.

(SEAL) Attest:

EDWARD. M.FLETCHER,JR. RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Pate] 

2. A process according to claim 1 in which said phosphoric acid is the crude acid coming from natural phosphate attack by the wet process.
 3. A process according to claim 1 in which an alkali metal phosphate in concentrated solution is introduced into the cathodic compartment, whereby as a result of the process the alkali metal to phosphorus ratio is enhanced.
 4. A process according to claim 1 in which a mixture of chlorine and a solution of alkali metal chlorides, is continuously removed from said anodic compartment.
 5. A process according to claim 1 in which a mixture of hydrogen and of phosphates in concentrated state is continuously removed from said cathodic compartment.
 6. A process according to claim 1 in which said cationic permselective membrane is made of a polymer presenting a good resistance to chlorine and alkaline agents.
 7. A process according to claim 1 in which said anode comprises a metal of the titanium group, coated only on one face with an active coating of a noble metal of the platinum group or of an oxide or alloy thereof. 